Thanks for your reply..  Could u please tell me what is b and v?  I guess v=Velocity right?  i am not an expertise in the subject.  But i have a clear understanding of gravity, velocity and acceleration.  I want to achieve my simulation like this..  All the debris after the blast (each debris being small objects) will move upward in the first instance slightly moving away from one another (i dont know how to calculate that) then ultimately falling down with gravity in the second instance.  So can i achieve both these instances with your suggested formula...  Fd= -m*g; Fu=-b*v; F=-m*g-b*v;  Actually i am not able to relate these formula with the required simulation.